Bronze is commonly defined as an alloy of copper and tin. However, other metals can be used, defining different bronze alloy variations such as commercial bronze (copper, zinc), architectural bronze (copper, zinc, lead) or aluminum bronze (copper, aluminum, nickel). The color of the bronze depends on the composition of the different metals used in the production of the alloy. For instance, a copper-rich bronze alloy may have a reddish appearance whereas a tin-rich bronze alloy may have a silvery-white appearance. The golden appearance of bronze is the result of a certain metallic composition.
Bronze can be obtained as a solid alloy by pyrometallurgy or as a plated material. As a plated material, bronze made of copper and tin is traditionally obtained by using cyanide plating baths. Bronze can be deposited directly as an alloy if cyanide chemicals are used. The product is thus formed by co-depositing copper and tin as plating takes place. More particularly, cyanide plating solutions are used during the electroplating of coinage blanks to obtain a golden bronze alloy layer. For example, U.S. Pat. No. 4,579,761 (Ruscoe at al.) describes a method of making aureate colored coins, medallions and tokens and products so made. The product is electroplated with alkaline cyanide copper-tin plating bath and then introduced into an annealing furnace at a constant temperature. After a further cleaning treatment, Ruscoe et al. describe obtaining a product coated with a shiny gold colored bronze.
Almost all commercially available bronze plating operations use cyanide based plating solutions to obtain a gold like color metallic finish as such operations are relatively simple and well known. However, cyanide-based plating solutions are toxic and this toxicity can be long lasting and can pose health and safety risks to humans, animals and fish downstream from the source of unwanted spills and leaks. Disposal of the waste can be expensive and challenging since the chemicals used to destroy cyanides can also be toxic themselves.
Non-cyanide bronze can be obtained by plating processes using certain commercial formulations, but the results are usually poor because the plated products tend to come out in a reddish color, very much resembling copper, rather than goldish as one would often desire in the production of a bronze finish. The non-cyanide plating solutions tend to be unstable, expensive and difficult to control for consistent results and color. Non-cyanide bronze plating is thus an unpopular and rarely used technique, particularly when plating is done for large quantities of industrial products, such as coinage blanks.
Furthermore, in order to reduce the cost of coinage, pure metals such as nickel, copper or aluminum, and solid alloys such as cupronickel, cartridge brass or aluminum bronze are being replaced gradually with coins made of a less expensive material such as steel for the core, plated over with nickel, copper and bronze in a single layer, double layer or triple layer as outer layers covering the steel core. The steel for the core is sometimes replaced with zinc, or copper, or a low cost alloy such as cartridge brass as variations of the process. U.S. Pat. Nos. 5,151,167 and 5,139,886 describe coins coated with nickel, copper and then nickel and a process for making such coins with the use of non-cyanide plating solutions. These patents disclose that the resulting coins have a regular surface exempted of surface pinholes, which is normally a problem associated with successive metals electroplating followed by annealing diffusion. The use of non-cyanide plating solution has thus been described as feasible in the successive coating of nickel, copper and nickel. Brass is also made by plating copper, followed by plating zinc on top of copper with non-cyanide plating solutions. The successive deposition of copper and zinc is followed by diffusion of zinc into copper at high heat and temperature to obtain a brass alloy. This type of non-cyanide brass alloy production is commercially performed at the Royal Canadian Mint. However, unlike zinc in brass diffusion, tin does not easily diffuse into a copper matrix because of tin's low melting point. Thus, the production of golden bronze with the combination of copper and tin has various different challenges compared to production from copper and zinc.
A general method for plating various alloys without the use of cyanide solutions is described in U.S. patent application published under No. 2006/0286400 (McDaniel et al.). McDaniel et al. describe a method that includes the steps of electroplating a layer of a first metal onto a substrate, electroplating a second layer of a second metal onto the first electroplated layer, and heating the combination of the substrate to produce an alloy finish including a bronze alloy.
Multiply-electroplating methods to produce golden bronze without the use of cyanide solutions are disclosed in international patent application published under No. WO2012/075572 (Nguyen et al.). Nguyen et al. disclose a multiple-plating method including plating a substrate with at least one copper layer and a tin layer provided with a certain relative thickness ratio, and annealing the plated substrate at a gradually increasing temperature to produce an inter-diffused outer bronze layer having a golden appearance.
However, there remains a need for techniques that provide enhanced production of articles having a golden bronze finish.